The Cretaceous continental sedimentation in SW Mongolia was commonly accompanied by volcanic activity. Bimodal volcanic association of the Cretaceous Khulsan Gol Formation is represented mostly by calc-alkaline and rare alkaline volcanic rocks (basaltic andesite, trachybasalt to trachyandesite lavas or tuffs with lahars). The lavas were extruded mainly subaerially, although locally there is a subaqueous component present corresponding to final phases of Mesozoic sedimentation in the Gobi-Altay rift zone. Variations in chemical and isotopic composition reflect crustal assimilation coupled with fractional crystallization of parental magma generated by melting of lower lithosphere and/or uppermost asthenospheric mantle. These within-plate volcanic rocks can be explained as initial stages of Cenozoic magmatic events in the Mongolia.